1. Field of the Invention
This invention relates to the structure of thin film magnetic read heads. More specifically, the invention relates to the improvement of sensitivity in giant magnetoresistive sensors.
2. Description of the Related Art
FIG. 1 (Prior Art) is a partial cross sectional view of a thin film read/write head combination. A read head 104 employing a giant magnetoresistive sensors 106 (hereinafter referred to as a “GMR sensors”) is combined with an inductive write head 102 to form a combined magnetic head 100. In a magnetic disk or tape drive an air bearing surface (ABS) of the combined magnetic head is supported adjacent to the moving magnetic media to write information on or read information from a surface of the media. In a write mode, information is written to the surface by magnetic fields that fringe across gap 114 between first 112 and second 116 pole pieces of the write head 102. Write head 102 also comprises yoke 120, coil 118, and backgap 122. In a read mode, the resistance of the GMR sensor 106 changes proportionally to the magnitudes of the magnetic fields from the moving magnetic media. When a sense current is conducted through the GMR sensor 106, resistance changes cause potential changes that are detected and processed as playback signals.
FIG. 2 (Prior Art) is an air bearing surface view 200 of read head 104 of FIG. 1. GMR sensor 106 includes a nonmagnetic conductive layer 206, also called a spacer layer, sandwiched between first and second ferromagnetic layers, referred to as a pinned reference layer 208, and a free layer 204. The magnetization of the pinned reference layer 208 is maintained (“pinned”) at 90 degrees to the magnetization of the free layer 204 by exchange coupling with pinned layer 212 and anti-ferromagnetic layer 214. The magnetization of the free layer 204 changes freely in response to magnetic fields from the moving magnetic media at the air bearing surface. When the directions of magnetization of the pinned and free layers are parallel, scattering of conduction electrons passing through the layers is minimal, and when the directions are antiparallel, scattering is maximized. Changes in the scattering of the conduction electrons change the resistance of the GMR sensor in proportion to sinθ, where θ is the angle between the magnetizations of the pinned and free layers. Sense DC current IDC 150 is conducted through the GMR sensor for detecting a change in resistance of the layer structure. This configuration of GMR sensor is typically known as a CPP-GMR sensor, which employ a sense current perpendicular to the plane of film layers. The change of resistance of the layer structure produces a voltage Vsense 152 which is directed to the monitoring electronics.
The anti-ferromagnetic layer 214 interfacially engages the pinned layers 212 and 208 in order to pin the magnetization of the pinned layers in a predetermined direction by magnetic exchange coupling. Since the anti-ferromagnetic pinning layer is not magnetized, it exerts no magnetic influence on the free layer 204. This is advantageous since the magnetization of the free layer should be free to rotate about a bias point in response to magnetic fields from the moving magnetic media. Advantageously, the magnetization of the pinned layers 212 and 208 can be strongly pinned by the anti-ferromagnetic pinning layer 214 so that their orientation cannot be easily changed by stray magnetic fields.
Structurally, the read head 104 includes a GMR sensor 106 sandwiched between shield layers 108 and 110. GMR sensor 106 typically comprises a 30 angstrom NiFeCr seed layer 216; a 150 angstrom PtMn anti-ferromagnetic pinning layer 214 grown on the seed layer 216; a CoFe pinned layer 212 grown on layer 214; a 4-8 angstrom Ru layer 210; a second CoFe pinned reference layer 208 grown on layer 210; a Cu 20-40 angstrom spacer layer 206 grown on pinned layer 208; a CoFe/NiFe 30 angstrom free layer placed on spacer layer 206; and, a Ta interface layer 202 grown between the free layer 204 and Shield 2 ref 110.
U.S. Pat. No. 5,695,864 discloses a device in which electrons flow through a free or excitable magnet, or reflect from it, to make its magnetization respond. To accomplish this, the spin vectors of the flowing electrons are preferentially polarized by an auxiliary ferromagnet, whose moment orientation is fixed. The electrons flow between the fixed and free ferromagnets through a non-magnetic metallic spacer which is thick enough to make the static inter-magnetic exchange coupling negligible. While transmitting through or reflecting from the free ferromagnet, the spins of the moving electrons interact by quantum-mechanical exchange with the local, permanently present, spontaneously-polarized electron spins of the free magnet. This interaction causes a transfer of vectorial angular momentum between the several metallic layers in the device which causes the magnetization vector of the free magnet to change its direction continually with time. Thus excited, the magnetization vector will precess about its original axis. The precession cone angle will either attain a new equilibrium value which will be sustained by the current or will increase beyond 90 degrees and precess with decreasing amplitude until the magnetization vector has reversed by 180 degrees from its initial direction.
U.S. Pat. No. 5,780,176 discloses an exchange coupling film having a stacked-film-structure consisting of a ferromagnetic film made of at least one material of Fe, Co and Ni, and an anti-ferromagnetic film. The exchange coupling film is made of a ferromagnetic material to which an element is added, provided at the interface between the ferromagnetic film and the anti-ferromagnetic film so as to improve the lattice matching. This results in the enhancement of the exchange coupling force. A magnetoresistance effect element including an exchange coupling film described above, and an electrode for supplying a current to the ferromagnetic film constitutes the exchange coupling film.
U.S. Pat. No. 5,919,580 discloses a spin valve device containing a chromium or chromium and aluminum anti-ferromagnetic layer, which acts as a pinning layer for a magnetoresistive ferromagnetic layer, by exchange coupling. The anti-ferromagnetic layer has a tunable Neel temperature and anisotropy constant, and is corrosion resistant.
U.S. Pat. No. 6,105,237 discloses a spin valve sensor provided with a spacer layer sandwiched between a free layer and a pinned layer. The pinned layer is pinned by a pinning layer constructed of a material having a high coercivity, and a low magnetic moment. The high coercivity is employed for pinning the pinned layer, and the low moment assures that stray fields from the pinning layer do not affect the coercivity of the free layer. The magnetic moment is preferably less than 300 emu/cc and the coercivity is preferably greater than 500 Oe. The magnetic orientation of the pinning layer is set by a magnetic field at room temperature that may be applied at the suspension level. The materials with which the pinning layer may be formed are amorphous materials TbFeCo and CoSm, and a non-amorphous material CoPtCr, provided the Cr is of sufficient proportion to minimize the moment of the CoPtCr material.
US Patent Application Publication US 2003/0151407 discloses a structure and method for forming a magnetic-field sensor device comprising depositing a first electrode onto a substrate. Then, an electrically insulating layer is deposited on the first electrode. Next, a portion of the insulating layer is removed to expose a region of the first electrode, thereby creating an empty space. After this, at least one layer of chemically-synthesized nanoparticles is deposited on the insulating layer and within the empty space. Next, a second electrode is deposited on both the layer of nanoparticles and the insulating layer. Alternatively, multiple layers of nanoparticles may be deposited, or only a single nanoparticle may be deposited. The substrate is either conducting or non-conducting, and the first and second electrodes are electrically conducting and may be magnetic or non-magnetic. Additionally, a metallic layer of magnetic material may be first deposited on the substrate.
US Patent Application Publication US 2004/0161636 discloses a structure and method of fabricating a magnetic read head, comprising forming a fill layer for a magnetic read head gap using atomic layer deposition (ALD). The fill layer comprises an insulator, preferably aluminum oxide, aluminum nitride, mixtures thereof and layered structures thereof. Materials having higher thermal conductivity than aluminum oxide, such as berylium oxide and boron nitride, can also be employed in layers within an aluminum oxide structure. The thickness of the ALD-formed head gap fill layer is between approximately 5 nm and 100 nm, preferably between approximately 10 nm and 40 nm.
In an article entitled “Control of Magnetization Dynamics in Ni81Fe19 Thin Films Through the Use of Rare Earth Dopants”, by Bailey et al., (IEEE Transactions on Magnetics, Vol. 37, No. 4, July 2001, pg 1749), the magnetization dynamics of soft ferromagnetic thin films tuned with rare earth dopants is disclosed. Low concentrations (2 to 10%) of Tb in 50 nm Ni81Fe19 films are found to increase the Gilbert magnetic damping parameter alpha over two orders of magnitude without great effect on easy axis coercivity or saturation magnetization.
One way to increase the sensitivity of the GMR sensors described above is to increase IDC, which increases Vsense for a given magnetic signal amplitude. However, increasing IDC beyond a certain point creates spin transfer torques which produce gross instability in the magnetization of the free layer. This instability is manifest as oscillations in the magnetization of the free layer, which are large enough in amplitude to obscure the magnetization changes induced by the moving media. The instability of the free layer induced by the spin transfer torques of the sense current, produces a type of noise which can be called spin transfer induced noise. It is noise because it produces a signal containing random fluctuations that can obscure the measurement signal of interest. It is to be distinguished from other types of noise such as thermal noise. The spin transfer induced noise significantly limits the sensitivity of prior art CPP-GMR sensors. What is needed is an improved GMR sensor having improved sensitivity and stability.